Fitted-Fabric Grainstones — Evidence for Vadose Diagenesis?

Abstract

Some carbonate grainstones have fitted fabrics that may form in the vadose zone and are therefore evidence for subaerial exposure. These grainstones may be composed of ooids, coated grains, skeletal grains or peloids. The grainstones have interlocking grains that fit together like puzzle pieces and could not possibly have formed that way during deposition. There is commonly isopachous rim cement surrounding the interlocking grains. The cement appears to have formed after the grains were fitted to each other but still fill a gap between the fitted grains. Examples of these fitted fabric grainstones have been identified in Devonian, Mississippian, Permian, Jurassic, Cretaceous, and modern strata of various parts of the world. This feature appears to be quite common if also commonly overlooked. It almost certainly occurs in carbonates throughout the geologic record. The fitted fabric was initially recognized and interpreted to be a result of vadose diagenesis by Dunham. Modern beachrock from the intertidal zone was noted to have grains that were more fitted to each other than grainstones forming in subtidal settings. Dissolution is interpreted to occur at grain contacts which over time flattens the contact and leads to the grains “fitting” to each other. There may be some pillars of undissolved material that preserve gaps between the grains that are later filled in with isopachous rim cement. The rocks have the appearance of being compacted and indeed this could be called “vadose compaction.” Burial compaction can be ruled out because there are few pressure solution features and the isopachous rim cement clearly postdates the fitting of the grains. Recognition of this important feature can help to identify cryptic exposure surfaces and sequence boundaries that otherwise might be difficult to recognize.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015